Description of Related Art
Capacitively coupled plasma (CCP) reactors arc utilized in a wide range of applications in the semiconductor device fabrication industry. CCP reactors have attracted great interest owing to their numerous advantages such as (1) a low aspect ratio of the plasma reactor due to a narrow gap between the cathode and the anode electrodes, (2) better radial uniformity of the plasma, (3) ease of plasma ignition, and (4) ability to use a shower-head-type planar gas introducing device to yield a better gas distribution.
One of the problems with CCP reactors concerns the nonuniform erosion of a gas inlet plate of the shower-head-type planar gas introducing unit. The gas inlet plate has a plurality of gas inlet holes which are subjected to a higher erosion rate by the plasma compared to other areas of the gas inlet plate. The erosion of the gas inlet holes occurs from a lower side as well as from an upper side thereof. The erosion process is explained in detail in accordance with FIGS. 4 and 5 below.
FIG. 4 shows a simplified diagram of one example of a CCP reactor 100. This CCP reactor 100 includes a top plate 51. a bottom plate 52, a cylindrical side wall 53, a top electrode 54 and a wafer holder 55. The top electrode 54 is arranged by a ring insulator 56 at the upper side of the CCP reactor 100 and the wafer holder 55 is arranged on the bottom plate 52 and supported by a planar insulator 57. Further, the insulator plate 58 is arranged between the top plate 51 and the top electrode 54. The top electrode 54 is made of a metal, for example, aluminum. Below the top electrode 54 there is a gas inlet plate 59. Between the top electrode 54 and the gas inlet plate 59, there is a narrow space 60 forming a gas reservoir. The purpose of the gas reservoir 60 is to provide a uniform gas distribution over the gas inlet plate 59. The material of the gas inlet plate 59 depends on the type of plasma application, for example, in dry etching applications, carbon or Si is usually used. In other applications, dielectric materials, for example, quartz or ceramic, are usually used. There is a large number of gas inlet holes 59a in the gas inlet plate 59 for the introduction of a process gas from the gas reservoir 60 to a plasma generation region. The diameter of the gas inlet holes 59a is approximately 0.5 mm. The separation between each of the gas inlet holes 59a may vary from 5 mm to 20 mm in an ordinary plasma source. However, regardless of the separation between the gas inlet holes 59a, an equal distance between the gas inlet holes 59a is usually kept. That is, gas inlet holes 59a are formed at the corners of identical squares drawn on the gas inlet plate 59. A wafer 61 to be processed is situated on the wafer holder 55. The wafer 61 faces the gas inlet plate 59 in a parallel fashion.
An RF power source 62 is connected to the top electrode 54 via a matching circuit 63. The RF power source 62 usually operates at a frequency in range of 10 MHz to 100 MHZ. When the RF electric power is supplied to the top electrode 54, a plasma is generated between the gas inlet plate 59 and the wafer holder 55 by a capacitively coupled mechanism. The plasma generation region, however, lies in the vicinity of the gas inlet plate 59, since an electron heating process primarily occurs by an oscillation of sheath voltage that lies just below the gas inlet plate 59. Therefore, a plasma density is higher, closer to the gas inlet plate 59 and gradually decreases in a downstream direction because of gas phase recombination and a ambipolar diffusion.
As mentioned above, a major problem associated with the gas inlet plate 59 is the high erosion rate of the gas inlet holes 59a by the plasma. This erosion results in a lower utilization efficiency of the gas inlet plate 59. The erosion process is explained below.
Plasma is usually generated at a low pressure, for example, in the range of 10 mTorr to 100 mTorr, in most present industrial applications. However, referring to FIG. 5, at an end 59a-1 of the gas inlet holes 59a there is a slightly higher gas pressure, and inside the gas reservoir 60 there is an even higher gas pressure. The plasma density changes depending on the pressure. At higher pressures, the plasma density becomes higher. In a capacitively coupled plasma, an RF electrode generally has a self bias voltage. In the situation explained above, the gas inlet plate 59 functions as the RF electrode and therefore, has a self bias voltage. The value and polarity of the self bias voltage generated in the gas inlet plate 59 depends on many parameters, for example, the surface area ratio of the cathode (the gas inlet plate) and the anode (all grounded surfaces where the plasma is in contact with), the operating frequency of the RF power source 62, and the plasma density, etc. In most of the plasma sources used in practical applications, the RF electrode attains a negative self bias voltage. Owing to this negative self bias voltage, the positive ions in the plasma accelerate towards the gas inlet plate 59 and bombard its surface. These ions gain a high energy by the acceleration process, thus the bombardment of ions on the gas inlet plate 59 causes a sputtering of the gas inlet plate 59. As explained above, the sputtering damage is higher at the gas inlet holes 59a, since there is a higher plasma density at these places. This process causes an extruded erosion of the gas inlet holes 59a compared to the other areas of the gas inlet plate 59, resulting in an enlargement of the diameter of the gas inlet hole 59a. With the increase of the gas inlet hole diameter, the plasma tends to confine in the gas inlet hole 59a by multireflections of electrons on the walls of gas inlet hole 59a. Accordingly, the erosion rate in the gas inlet hole 59a accelerates with the plasma-on time. This process yields a conical shaped gas inlet hole 64 at the lower end 59a-1 thereof, as shown in FIG. 5, after several hours of operation.
Similarly, micro-plasmas generated at an upper end 59a-2 of the gas inlet hole 59a causes an erosion of the upper side of the gas inlet hole 59a. Because of these erosion processes, a conical shaped gas inlet hole 65 is formed at the upper end 59a-2 thereof, as shown in FIG. 5. The service life of the gas inlet plate 59 is limited by the condition that the eroded upper and lower ends of the gas inlet hole 59a are joined. As a result, usually thicker, approximately 10 mm, gas inlet plates are used with CCP reactors 100 in order to increase their service life. However, the gas inlet plate 59 has a very low utilization efficiency since the erosion of gas inlet holes 59a determines its service life.
Moreover, if a polymer deposition gas chemistry is used in generating the plasma, a polymer deposition 66 can be observed in an area on the lower surface of the top electrode 54, which is just above the gas inlet hole 59a as shown in FIG. 5. The polymer deposition 66 can be attributed to the following two reasons. (1) Due to the micro-plasma generated at the upper end 59a-2 of gas inlet hole 59a, polymer depositing radicals are formed. (2) The polymer depositing radicals generated in the main plasma below the gas inlet plate 59 can diffuse through the gas inlet hole 59a. This diffusion process increases with an increase of the gas inlet hole diameter with plasma-on time. Before the thickness of the layer formed by the polymer deposition 66 becomes thick enough to peel off and release into the plasma as micro-particles, the processing of the wafer 61 based on the plasma in the CCP reactor 100 should be stopped. Existence of the micro-particles in the plasma causes faulty devices.